Obesity is a major public health issue. SIM1 is one of six known human genes that when mutated cause severe obesity. SIM1 encodes a basic helix-loop-helix-PAS (bHLH-PAS) transcription factor essential for the developmental program of certain regions of the hypothalamus, including the paraventricular nucleus (PVN). SIM1 is also expressed post-developmentally in the PVN, where it may act physiologically to regulate food intake. Sim1 (heterozygous) knockout mice, in which the dosage of Sim1 is reduced by half (haploinsufficiency), eat more food than controls, develop obesity, show increased linear growth, and do not show a normal reduction in the mass of food they consume when challenged with a calorically dense, high fat diet. All of these abnormalities are also seen in mice with mutations that interfere with hypothalamic signaling via the melanocortin 4 receptor (MC4R), which is abundantly expressed in neurons within or projecting to the PVN. MC4R mutations are the most common genetic cause of severe, early onset human obesity. Sim1 mice show defective activation of PVN neurons in response to a melanocortin agonist. Key questions remain unanswered regarding how Sim1 haploinsufficiency results in defective Mc4r signaling and how these changes relate to the associated hyperphagia and obesity. Are the metabolic effects of Sim1 haploinsufficiency a consequence of aberrant hypothalamic development, or are they secondary to yet-to-be-characterized post-developmental actions of the transcription factor? Which PVN neurons are responsible for the hyperphagia of Sim1 mice? If the effects of Sim1 on feeding behavior are due to its actions in adults, genes that are transcriptionally regulated by Sim1 become potential targets for therapeutic interventions to reduce hyperphagia and diet-induced obesity. The aims of this proposal address the mechanism of hyperphagic obesity and impaired Mc4r signaling in Sim1 mice. The first aim will determine whether PVN neurons in Sim1 mice have normal cell numbers, hindbrain projections, and Mc4r expression. The second aim tests directly whether Sim1 acts post-developmentally to cause hyperphagic obesity. The final aim tests whether Sim1 regulates feeding behavior by acting in specific subsets of PVN neurons expressing oxytocin, Trh, or Crh. Completion of these aims will facilitate translating the discovery of SIM1 as a monogenic obesity gene into improvements in prevention and treatment of common obesity and its associated morbidity.